The present invention relates to wireless communication receivers, and particularly relates to demodulating amplitude-modulated signals.
Higher-order modulation constellations represent one mechanism supporting the faster data rates of current and evolving wireless communication systems. For a given symbol rate, more bits in each modulation symbol translate into higher effective data rates, assuming acceptable demodulation performance at the receiver. In general, the use of more complex modulation constellations for data transmission results in greater demodulation challenges at the receivers.
In part, such challenges arise because higher order modulation schemes commonly use amplitude changes to convey information. For example, Quadrature Amplitude Modulation (QAM) generally defines modulation constellation points arrayed around the origin in the real-imaginary plane, where each point represents a unique pairing of phase and amplitude. 16 QAM, for example, defines four points in each quadrant, yielding sixteen points in the QAM constellation. Each such point represents one of sixteen modulation symbols and is recognizable at the receiver based on the unique pairing of phase and amplitude represented by the symbol. With sixteen unique symbols, each QAM modulation symbol represents four bits, and thus affords higher effective data rates than QPSK, for example, assuming the same modulation symbol rate.
Sometimes different modulation formats are used at different times, with the choice often made as a function of available transmitter resources, prevailing radio conditions, and/or required data rates. For example, the High Speed Downlink Packet Access (HSDPA) extension of the Wideband CDMA (W-CDMA) standard offers a range of data rates on a High Speed Downlink Shared Channel (HS-DSCH), which is a shared packet data channel using either QPSK or QAM modulation, depending on data rate requirements.
Use of 16QAM on the HS-DSCH enables higher data rates, but such use complicates data reception. Specifically, an amplitude reference is required at the receiver to detect received symbols and properly scale the soft information for decoding (e.g., by a turbo decoder). For HS-DSCH, and for CDMA-based pilot-and-traffic-channel transmissions in general, the needed amplitude reference represents the relative scaling between the code channel used for estimation (e.g., the pilot or reference channel) and the code channel(s) being demodulated (e.g., the traffic or information-bearing channels). The needed scale factor g can be written ashtraf=ghpil  Eq. 1where hpil is a channel response vector, as defined in G. E. Bottomley, T. Ottosson, and Y.-P. E. Wang, “A Generalized RAKE receiver for interference suppression,” IEEE J. Select. Areas Commun., vol. 18, pp. 1536-1545, August 2000, for example, which can be estimated from the pilot channel, htraf represents the channel response vector for the traffic channel, and g is the scale factor. To illustrate the usefulness of the scale factor, consider the Log-Likelihood Ratio (LLR) for bit bj of the ideal QAM demodulator, which is given as
                              LLR          ⁡                      (                          b              j                        )                          =                                            ∑                                                s                  i                                ∈                                                      S                    0                                    ⁡                                      (                    j                    )                                                                        ⁢                          exp              ⁢                              {                                  γ                  ⁡                                      (                                                                  2                        ⁢                                                  Re                          ⁡                                                      (                                                                                                                            s                                  i                                  *                                                                ⁢                                z                                                                                                                              w                                  H                                                                ⁢                                                                  h                                  traf                                                                                                                      )                                                                                              -                                                                                                                              s                            i                                                                                                    2                                                              )                                                  }                                                                        ∑                                                s                  i                                ∈                                                      S                    1                                    ⁡                                      (                    j                    )                                                                        ⁢                          exp              ⁢                              {                                  γ                  ⁡                                      (                                                                  2                        ⁢                                                  Re                          ⁡                                                      (                                                                                                                            s                                  i                                  *                                                                ⁢                                z                                                                                                                              w                                  H                                                                ⁢                                                                  h                                  traf                                                                                                                      )                                                                                              -                                                                                                                              s                            i                                                                                                    2                                                              )                                                  }                                                                        Eq        .                                  ⁢        2            
In the above equation, z is the estimated symbol, si is a candidate symbol from a normalized scale constellation, γ is the signal-to-noise ratio (SNR), which actually may be calculated as a signal-to-interference-plus-noise ratio (SINR), and w represents a vector of combining weights. Assuming a log-max turbo decoder, the log-likelihood ratio becomes
                              LLR          ⁡                      (                          b              j                        )                          =                              γ            ⁡                          [                                                                    max                                                                  s                        i                                            ∈                                                                        S                          0                                                ⁡                                                  (                          j                          )                                                                                                      ⁢                                      (                                                                  2                        ⁢                                                  Re                          ⁡                                                      (                                                                                                                            s                                  i                                  *                                                                ⁢                                z                                                            μ                                                        )                                                                                              -                                                                                                                              s                            i                                                                                                    2                                                              )                                                  -                                                      max                                                                  s                        i                                            ∈                                                                        S                          1                                                ⁡                                                  (                          j                          )                                                                                                      ⁢                                      (                                                                  2                        ⁢                                                  Re                          ⁡                                                      (                                                                                                                            s                                  i                                  *                                                                ⁢                                z                                                            μ                                                        )                                                                                              -                                                                                                                              s                            i                                                                                                    2                                                              )                                                              ]                                .                                    Eq        .                                  ⁢        3            The normalization factor μ in Eq. 3 is defined asμ=wHhtraf.  Eq. 4Here, the normalization factor scales the symbol estimate z for comparison to the symbols si from a unitary average power constellation. Conventionally, receivers determine the scale factor through time estimation of the RMS value of the symbol estimate, which may be expressed as,√{square root over (|z|2)}≈wHhtraf=μ.  Eq. 5From the above equations, one sees that the normalization factor provides a basis for properly evaluating the log-likelihood ratio used in decoding the estimated traffic symbol z. It may be observed that Eq. 4 can be rewritten in consideration of Eq. 1 asμ=gwHhpil.  Eq. 6Thus, as an alternative to calculating a normalization value from Eq. 5, received signal processing may be configured to the normalization factor given in Eq. 6. However, that calculation may be challenging in terms of implementing it efficiently and/or integrating the calculation into existing received signal processing calculations.